1. Field of the Invention
The present invention relates to ultrasonic sensors and methods for manufacturing the ultrasonic sensors, and more particularly, to an ultrasonic sensor included in, for example, a backup sensor of an automobile.
2. Description of the Related Art
FIG. 4 is a diagram illustrating an example of a known ultrasonic sensor. An ultrasonic sensor 1 includes a cylindrical casing 2 having a bottom portion and made of aluminum or other suitable material. An inner bottom surface of the casing 2 is bonded to a surface of a piezoelectric element 3 at one side thereof. An inner space of the casing 2 is substantially entirely filled with foamable resin 4, such as foamable silicone, so that the piezoelectric element 3 is covered with the foamable resin 4. In addition, a substrate 6 having terminals 5a and 5b is attached to an opening portion of the casing 2 so as to cover the foamable resin 4. Electrodes 7a and 7b, which are respectively connected to the terminals 5a and 5b, are provided on either side of the substrate 6. The terminal 5a is connected to a surface of the piezoelectric element 3 at the other side thereof through the electrode 7a provided on an inner surface of the substrate 6 and a wire 8. The terminal 5b is connected to the surface of the piezoelectric element 3 at the one side thereof through the electrode 7b on an outer surface of the substrate 6, solder 9, and the casing 2.
When measuring a distance to an object using the ultrasonic sensor 1, the piezoelectric element 3 is excited by applying a drive voltage to the terminals 5a and 5b. The bottom surface of the casing 2 is vibrated in response to vibration of the piezoelectric element 3. As a result, ultrasonic waves are emitted in a direction substantially perpendicular to the bottom surface, as indicated by an arrow in FIG. 4. When the ultrasonic waves emitted by the ultrasonic sensor 1 are reflected by the object and return to the ultrasonic sensor 1, the piezoelectric element 3 is vibrated. The vibration of the piezoelectric element 3 is converted into an electric signal, and the electric signal is output from the terminals 5a and 5b. The distance between the ultrasonic sensor 1 and the object can be determined by measuring the time from when the drive voltage is applied to when the electric signal is output.
In the ultrasonic sensor 1, vibration of the overall body of the casing 2 is suppressed because the inner space of the casing 2 is filled with the foamable resin 4. Also, ultrasonic waves that are emitted toward the inside of the casing 2 are dispersed and absorbed by the large number of pores in the foamable resin 4. Thus, vibration of the casing 2 itself and the ultrasonic waves remaining in the casing 2 can both be efficiently reduced and reverberation characteristics can be improved (see Japanese Unexamined Patent Application Publication No. 11-266498).
Since the ultrasonic sensor 1 includes the terminals 5a and 5b, the ultrasonic sensor 1 can be mounted by automation. However, since the substrate 6 including the terminals 5a and 5b is attached to the casing 2 such that the substrate 6 is in direct contact with side surfaces of the casing 2, vibration of the piezoelectric element 3 is transmitted through the casing 2 and the substrate 6 and is damped through the terminals 5a and 5b. 
FIG. 5 is a diagram illustrating an example of a new ultrasonic sensor that provides a basis for the present invention. In an ultrasonic sensor 1′ shown in FIG. 5, in contrast to the ultrasonic sensor 1 shown in FIG. 4, a disc-shaped substrate 6a including terminals 5a and 5b is not attached to a casing 2 such that the substrate 6a is in direct contact with the casing 2. Instead, the substrate 6a is fitted in a hole provided at the approximate center of a damping member 6b that is made of silicone rubber and that is fitted over an opening portion of the cylindrical casing 2 having a bottom portion. Thus, the substrate 6a is attached such that the substrate 6a is in contact with the foamable resin 4. The terminal 5a is connected to a piezoelectric element 3 through a wire 8a, and the terminal 5b is connected to the piezoelectric element 3 through a wire 8b and the casing 2.
In the ultrasonic sensor 1′ shown in FIG. 5, the substrate 6a is not in direct contact with the casing 2. Therefore, transmission of vibration from the piezoelectric element 3 to the substrate 6a and the terminals 5a and 5b through the casing 2 is suppressed by the damping member 6b. In other words, in the ultrasonic sensor 1′, vibration of the piezoelectric element 3 is not easily transmitted to the substrate 6a or the terminals 5a and 5b, and is not easily damped.
To perform automated mounting, the terminals must have extremely high positional accuracy. However, in the ultrasonic sensor 1′ shown in FIG. 5, the substrate 6a including the terminals 5a and 5b is fitted in the hole provided at the approximate center of the damping member 6b. Therefore, the perpendicularity of the terminals 5a and 5b with respect to the casing 2 and the piezoelectric element 3 is degraded and the positional accuracy of end portions of the terminals 5a and 5b with respect to the casing 2 and the piezoelectric element 3 is reduced.
In addition, in the ultrasonic sensor 1′ shown in FIG. 5, if an external stress is applied after the ultrasonic sensor 1′ is mounted, for example, if a top surface (surface at the piezoelectric-element-3 side) is pushed from the outside, the foamable resin 4, which is soft, is severely deformed. As a result, large stress and displacement may occur in areas where the terminals 5a and 5b are connected to the lead wires 8a and 8b, respectively. This may lead to defects, such as disconnection, for example.